The present invention relates to a process and apparatus for separating aspherical particles from spherical particles, and more especially, to a process and apparatus for separating aspherical particles from spherical particles which both consist of organic or inorganic material, the spherical particles being used as the carrier material for the toner of electrophotographic developer substances.
It is conventional in electrophotographic processes to produce a latent image on a photoelectrically conductive recording material. This electrostatic image is made visible or developed by means of a toner, which is applied to the surface of the recording material. The toner particles must likewise be charged electrostatically with a polarity opposite to the charge polarity of the latent electrostatic image. To charge the surface of the toner electrostatically, the toner is mixed with a carrier material which is selected so that, by surface charging, the appropriate electric charge of the toner is produced when the toner and carrier are separated from one another. When this developer mixture is scattered onto the recording material by a suitable application system, the toner separates from the carrier material and, due to its opposite polarity, adheres to the recording material solely in areas where charges in the form of the latent electrostatic image are still present.
The image made visible or developed in this manner with the toner is then contact-transferred to a carrier, for example, a paper sheet. Subsequently, the toner image which is relatively loose on the paper sheet is fixed so as to render the image permanent.
Although images and/or copies of good quality are produced by means of the electrophotographic processes, such copies often have transfer defects in the form of blurred image areas as a result of non-uniform color density. These transfer defects may be caused by various reasons. One of these reasons is indirectly due to the carrier material mixed with the toner.
During development of a latent electrostatic image on the recording material, the individual carrier particles provided with toner roll over the recording material in response to gravitational forces. The recording material is arranged so that the carrier particles give up the toner particles transported with them to the still-existing electrostatic charge areas but themselves do not remain on the recording material. Instead, they roll on into a toner supply container. It frequently occurs, however, that individual carrier particles remain on the recording material. The result is that the carrier particles, which are much larger than the toner particles, form real elevations on the recording material. When the toner image is then transferred by direct contact for final use, for example, onto a paper sheet, intimate contact between the toner and the paper in the immediate vicinity of these carrier particles is then interrupted by the elevations. The paper which, as is known, is also statically charged, is thus not capable of attracting the toner particles arranged around the carrier particles. Missing image areas or non-uniform color density are the result of such remaining carrier particles. Transfer defects of this type become particularly disadvantageously noticeable when literature is copied, particularly literature with numerical tables and mathematical formulae. Therefore, these defects must be avoided.
As carrier materials, there have been suggested organic and inorganic materials. In practice, glass and iron have proven to be suitable inorganic materials. The carriers are used in particulate form having a preferable grain size of about 200 to 600.mu.. The average grain size of the toner, on the other hand, is only about 10 to 60.mu..
The ideal geometrical form of the carrier particles is that of a sphere. For practical and economical reasons, the carrier material, however, cannot be produced in such ideal grain configuration. Rather the carrier consists of a mixture of nearly spherical particles and of particles having random geometrical form. For the sake of simplicity, the nearly spherical particles are hereinafter called spherical particles and the particles deviating therefrom are called aspherical particles.
Of particularly great disadvantage are the aspherical particles contained in the mixture. They are present in the form of small rods, ellipsoids or the like, which additionally are provided with sharp points or sharp edges. In the hitherto known production processes, the approximate proportion of aspherical particles is about 15 to 30 percent.
Transfer defects can be prevented if it would be possible to completely separate the aspherical carrier particles from the spherical carrier particles. It has been found that the carrier particles which remain on the photoelectrically conductive recording material belong exclusively to the aspherical category. Because of their non-uniform surface, they tend to remain much more than the carrier particles of the spherical category.
Another not insignificant disadvantage results from the sharp edges and points of the aspherical carrier particles. During development of the latent image, they cause fine scratches on the surface of the recording material, and in the scratched areas electrostatic charging is no longer possible, and thus, the toner can no longer adhere there. The transfer defects resulting therefrom become particularly noticeable when the recording material has been used for the production of several hundred copies. Since the recording material, however, normally can be used for producing several thousand copies, it must be prematurely replaced and is not used to its fullest economic extent. Even those carrier particles which for certain reasons are often additionally provided with a fine coating of plastic are no exception, because the plastic material does not adhere to the sharp points and edges.